In a process for manufacturing semiconductor devices, various plasma processing such as etching, film forming by sputtering or CVD film forming (Chemical Vapor Depositing) have been frequently employed.
There have been known various types of plasma processing apparatuses, among which a capacitive coupling type parallel plate plasma processing apparatus is the apparatus the most popularly distributed and used.
This type of plasma processing apparatus has a pair of parallel flat plate electrodes (upper and lower electrodes) in a reduced-pressure chamber. A semiconductor wafer to be processed is placed between the electrodes, then process gas (treatment gas) is introduced into the chamber and electric power with high frequency is applied to one of the electrodes. A high frequency electric field generated between the electrodes generates plasma from the process gas to perform plasma processing on the semiconductor wafer.
In etching an oxide film or the like formed on a semiconductor wafer with use of such a plasma processing apparatus, the pressure in the chamber is set at an intermediate level to generate plasma having an inter-mediate density, thereby the optimum radical control can be attained. In such a condition, the optimum plasma can be obtained to realize etching with good stability and reproducibility at a high selective ratio.
In accordance with the downsizing of a device, however, the request for the ultra-integration of a circuit is increased. There are also design rule constraints on features like the contact hole. The contact hole is required to be thin and deep, i.e., to have higher aspect ratio. The conventional oxide film etching method is, therefore, beginning to be not good enough to satisfy the demands of the market.
To cope with such recent requests, the frequency of the electric power applied to the electrode is set at a higher level to generate plasma having a higher density, so as to maintain good dissociation. By generating the plasma in such a manner, suitable plasma can be generated in a lower pressure, and thus the device with a smaller design rule can be manufactured.
With the conventional plasma processing apparatus, however, the upper electrode is formed from a conductor or semiconductor. Therefore, when the frequency of the electric power applied to the upper electrode is set at a high level, the inductance on the surface of the electrode will be increased so as not to be neglected, whereby the electric field in the opposite direction will be uneven.
Further, such a higher density of the plasma by the higher frequency remarkably causes non-linear characteristics of the plasma, so that a harmonic may be easily interposed on the reflected wave form the plasma. Particularly, with use of the electrode having a diameter of 250 to 300 mm, it has been found from experience that such a harmonic generates a standing wave on the surface of the electrode, which makes the electric field on the surface of the electrode uneven.
If the electric field is made uneven in such a manner, the density of plasma will be also made uneven, with the result that the etching rate of etching will be uneven. Accordingly, it is essential to make the etching rate even by eliminating the causes of the uneven electric field.
The above-mentioned problems in generating a high-density plasma, however, have not been recognized clearly, and thus a proposal for preventing the above-mentioned uneven electric field has not sufficiently been presented yet.
Further, according to the conventional plasma processing apparatus, the electric power is applied to the upper electrode with use of an electric power applying rod, and thus a box having a size substantially equal to a chamber encloses the electric power applying rod to shield electromagnetic wave.
However, since the inductance of the electric power applying rod is very high, if the frequency of the high frequency power supplied to the upper electrode is set at a higher level in order to increase the plasma density, the harmonic of the wave reflected from the plasma is reflected due to the inductance component of the electric power applying rod. Further, reflection is caused at every portions within the box in which the electric power applying is disposed, and the resultant reflected harmonic backs to the surface of the upper electrode exposed to the plasma.
With the electrode having a diameter of 250 mm to 300 mm, a standing wave will be easily generated on the surface of the electrode due to the higher harmonic (higher harmonic), which makes the electric field on the surface of the electrode uneven.
The electric power applying rod is provided to the center of the upper electrode on the rear surface thereof. When the frequency of the electric power applied to the electrode is increased to generate high-density plasma, the high frequency current flows only on the surface of the electrode. The high frequency electric power applied from the electric power applying rod to the upper electrode flows through the rear surface of the electrode to the outer periphery of the round electrode to be supplied from the outer periphery to the center of the electrode.
The outer periphery of the electrode is enclosed by an insulator (capacity component) and the chamber enclosing the insulator is grounded. With this structure, the standing wave is generated on the plasma contacting face of the upper electrode by the interference, which makes the electric field on the electrode in the direction of the diameter uneven. The unevenness of the electric field also makes the density of the plasma uneven, which causes an uneven etching rate. Accordingly, these causes need to be eliminated to make the etching rate even.
However, as mentioned before, the problems in generating the high-density plasma, have not been recognized clearly, and thus a proposal for preventing the above-mentioned uneven electric field has not been sufficiently presented yet.